Single-Fiber Bidirectional Controller Area Network Bus

ABSTRACT

A controller area network (CAN) comprising a plurality of CAN nodes that communicate via a CAN bus that comprises a fiber optical network. The fiber optical network uses a single fiber and a single wavelength for transmit and receive, and comprises a passive reflective optical star. The reflective optical star comprises an optical mixing rod having a mirror at one end. The other end of the reflective optical star is optically coupled to the transmitters and receivers of a plurality of optical-electrical media converters by way of respective high-isolation optical Y-couplers. Each CAN node produces electrical signals (in accordance with the CAN message-based protocol) which are converted into optical pulses that are broadcast to the fiber optical network. Those optical pulses are then reflected back to all CAN nodes by the reflective optical star.

BACKGROUND

The technology disclosed herein generally relates to optical networksthat enable communication between electrical components.

Data transmission between electrical components is typically achievedvia a network comprising electrical cables. In analog avionic systemsthe number of cables used to transfer information between the varioussystem components can be considerable. In digital systems, signals aretransmitted along a single pair of wires, which makes up a data bus. Abus is a collection of wires through which data is transmitted from onepart of a network to another. Bus systems provide an efficient means ofexchanging data between the diverse avionic systems onboard an aircraft.All buses consist of an address bus and a data bus. Typically aircraftbus systems use serial data transfer because it minimizes the size andweight of aircraft cabling.

In some scenarios, it is desirable to connect a number of linereplaceable units (LRUs) to each other. For example, a number of LRUs inthe forward section of a vehicle (e.g., an aircraft) may be connected toa number of LRUs in the aft section of the vehicle. Connecting each LRUto every other LRU could result in an unreasonably large number ofconnections between LRUs. If all of these connections are in the form ofcopper wires, the resulting space and weight of the connections could beburdensome for the vehicle. Electrical data buses have been used toconnect LRUs.

More specifically, it is known to use an electrical controller areanetwork (CAN) to connect electrical devices (also known as “CAN nodes”)to each other by way of a multi-master serial bus (also known as a “CANbus”). The devices that are connected by a CAN bus are typicallysensors, actuators, and other control devices. For example, it is knownto facilitate communication between a multiplicity of LRUs using anelectrical controller area network.

Current electrical CAN bus assemblies on an airplane have manyundesirable characteristics, including at least the following: (1)time-consuming assembly of T-couplers involving lifting and reconnectingthe double shields on a cable; (2) critical stub lengths from theT-coupler to the LRU prevent reuse of a bus assembly for another CAN buswith the same node quantity; (3) CAN simple electrical signaling is notwell protected from electro-magnetic effects (EME) as compared to Arinc629 doublets; (4) careful treatment of terminator/T-coupler andT-coupler/node interfaces is required to avoid impedance mismatch; and(5) CAN operational issues. such as beyond economic repair, impedancemismatch, and bus reflection become worse for longer airplanes.

A single optical data bus can eliminate some of the weight and size ofelectrical connections between LRUs. In general, optical communicationfibers, such as glass optical fibers and plastic optical fibers, can belighter and contained in smaller spaces than electrical wiring. Inparticular, optical networking using plastic optical fibers may provideadvantages over networking using copper or other metal wiring.Categories of plastic optical fiber include plastic-clad silicon opticalfiber, single-core plastic optical fiber, or multi-core plastic opticalfiber. Plastic optical fiber networking may have lower installation andmaintenance costs. Moreover, because plastic optical fibers are lighterthan the metal wiring that would be needed to carry an equivalent amountof data, using plastic optical fibers may result in appreciable weightsavings. The weight savings may be significant for networks onboardvehicles, such as aircraft, where the weight savings may result inreduced fuel consumption and lower emissions.

At present, no industry standard is defined for the fiber optictransmissions of CAN signals. There are industry publication, patentsand products for optical CAN bus but they use one or more of thefollowing: separate fibers for transmit and receive to maintainseparation of dominant bits and recessive bits; an active optical starwith single point failure of the hub; and a single-fiber bidirectionalflow of data from point to point (two-node bus) repeater for lengthextension with a dichroic mirror and two wavelengths.

It would be desirable to provide improvement to enhance the performanceof CAN buses comprising a fiber optical network.

SUMMARY

The broad concept disclosed herein for enhancing CAN bus performanceconverts an electrical CAN bus (e.g., ARINC 825) to a passive opticalCAN bus to reduce weight and labor associated with manufacturing andinstallation of the bus, and to minimize the many bus configurationsassociated with different data speeds and bus/stub lengths. The opticalCAN bus disclosed herein makes use of a single fiber for both transmitand receive, a single wavelength for both transmit and receive, apassive reflective optical star, and high-isolation optical Y-couplers(hereinafter “optical Y-coupler”).

More specifically, the subject matter disclosed in detail below isdirected to a controller area network (CAN) comprising a plurality ofCAN nodes that communicate via a CAN bus that comprises a fiber opticalnetwork. The fiber optical network uses a single fiber and a singlewavelength for transmit and receive, and comprises a passive reflectiveoptical star, which receives broadcast optical pulses and reflects themback toward all CAN nodes. The reflective optical star comprises anoptical mixing rod having a mirror at one end. The other end of thereflective optical star is optically coupled to the transmitters andreceivers of a plurality of optical-electrical media converters(including a plurality of transmit optical subassemblies and a pluralityof receive optical subassemblies) by way of respective high-isolationoptical Y-couplers. Each CAN node produces electrical signals inaccordance with the CAN message-based protocol, which electrical signalsare converted into optical pulses that are broadcast to the fiberoptical network. Those optical pulses are then reflected back to all CANnodes by the reflective optical star.

The reflective optical star comprises an optical mixer which isconnected to the transmitters and receivers respectively of a pluralityof optical-electrical media converters by way of a respective pluralityof optical Y-couplers. Each optical-electrical media converter comprisesa respective receiver optically coupled to one branch of a respectiveoptical Y-coupler by way of a respective output plastic optical fiberand a respective transmitter optically coupled to the other branch ofthe respective optical Y-coupler by way of a respective input plasticoptical fiber. Optionally, glass optical fibers may be used instead ofplastic optical fibers.

In accordance with one embodiment, the controller area network comprisesa respective signal converter that couples a respective CAN node to arespective transmit optical subassembly and a respective receive opticalsubassembly, which subassemblies in turn are optically coupled to thereflective star coupler by way of optical fibers and optical Y-couplers.In accordance with the example embodiments disclosed in detail below,the CAN nodes are incorporated in respective line replaceable units(LRUs) onboard an aircraft.

One aspect of the subject matter disclosed in detail below is a datacommunications system comprising: a plurality of controller area networknodes which operate electrically; a plurality of signal converterselectrically coupled to respective controller area network nodes of theplurality of controller area network nodes, each signal convertercomprising electrical circuitry that converts differential signals todigital signals and vice versa; a plurality of transmit opticalsubassemblies electrically coupled to respective signal converters ofthe plurality of signal converters, each transmit optical subassemblycomprising a respective transmitter that converts digital signals from arespective signal converter to optical pulses; a plurality of receiveoptical subassemblies electrically coupled to respective signalconverters of the plurality of signal converters, each receive opticalsubassembly comprising a respective receiver that converts opticalpulses to digital signals which are sent to a respective signalconverter; and a fiber optical network optically coupled to thetransmitters and receivers for enabling the plurality of controller areanetwork nodes to communicate with each other, wherein the fiber opticalnetwork comprises a reflective optical star. In accordance with someembodiments, the fiber optical network further comprises a plurality ofoptical Y-couplers optically coupled to the reflective optical star,each optical Y-coupler comprising transmit and receive branches whichare respectively optically coupled to the transmitter and receiverassociated with a respective signal converter. In one proposedimplementation, the transmit branch of each optical Y-coupler comprisesa first optical fiber having a first side face, the receive branch ofeach optical Y-coupler comprises a second optical fiber having a secondside face that confronts the first side face, and each optical Y-couplerfurther comprises a layer of reflective material disposed between thefirst and second side faces of the transmit and receive branches. Thefirst optical fiber has a first end face, the second optical fiber has asecond end face, and each optical Y-coupler further comprises a thirdoptical fiber having an end face which is optically coupled to the firstand second end faces. The reflective star coupler comprises an opticalmixing rod and a mirror disposed at one end of the optical mixing rod.

Another aspect of the subject matter disclosed in detail below is a datacommunications system comprising: a plurality of electrical devicesconfigured for sending and receiving electrical signals representingdata, wherein each of the electrical devices comprises a respectivecontroller area network controller configured to broadcast messagesusing bitwise arbitration to determine message priority, and arespective controller area network transceiver electrically coupled tothe respective controller area network controller; a plurality of signalconverters electrically coupled to respective controller area networktransceivers of the plurality of controller area network transceivers,each signal converter comprising electrical circuitry that convertsdifferential signals to digital signals and vice versa; a plurality oftransmit optical subassemblies electrically coupled to respective signalconverters of the plurality of signal converters, each transmit opticalsubassembly comprising a respective transmitter that converts digitalsignals from a respective signal converter to optical pulses; aplurality of receive optical subassemblies electrically coupled torespective signal converters of the plurality of signal converters, eachreceive optical subassembly comprising a respective receiver thatconverts optical pulses to digital signals which are sent to arespective signal converter; and a fiber optical network opticallycoupled to the transmitters and receivers for enabling the plurality ofcontroller area network nodes to communicate with each other, whereinthe fiber optical network comprises a reflective optical star. Inaccordance with one embodiment, each of the plurality of electricaldevices is a respective line replaceable unit.

A further aspect of the subject matter disclosed in detail below is adata communications system comprising: a plurality of electrical devicesconfigured for sending and receiving electrical signals representingdata, wherein each of the electrical devices comprises a respectivecontroller area network controller configured to broadcast messagesusing bitwise arbitration to determine message priority, and arespective controller area network transceiver electrically coupled tothe respective controller area network controller; means for convertingdifferential electrical signals to optical pulses; means for convertingoptical pulses to differential electrical signals; and a fiber opticalnetwork comprising a reflective optical star and a multiplicity ofoptical wave guides that optically couple the reflective optical star tothe means for converting differential electrical signals to opticalpulses and to the means for converting optical pulses to differentialelectrical signals. In accordance with some embodiments, themultiplicity of optical wave guides further comprise a plurality ofoptical Y-couplers optically coupled to the reflective optical star,each optical Y-coupler comprising transmit and receive branches whichare respectively optically coupled to a transmitter associated with arespective means for converting differential electrical signals tooptical pulses and a receiver associated with a respective means forconverting optical pulses to differential electrical signals.

Yet another aspect is a method for controller area network communicationbetween a plurality of nodes, comprising: broadcasting a message fromone of the plurality of nodes, the message comprising transmitdifferential electrical signals representing a sequence of bits inaccordance with a communication protocol that employs bitwisearbitration in response to collision detection; converting the transmitdifferential electrical signals to optical pulses; guiding the opticalpulses toward and into a reflective optical star; reflecting the opticalpulses inside the reflective optical star; guiding reflected opticalpulses toward the plurality of nodes; converting reflected opticalpulses to receive differential electrical signals; and receiving thereceive differential electrical signals at each node.

Other aspects of CAN buses comprising a fiber optical network aredisclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, functions and advantages discussed in the precedingsection can be achieved independently in various embodiments or may becombined in yet other embodiments. Various embodiments will behereinafter described with reference to drawings for the purpose ofillustrating the above-described and other aspects. None of the diagramsbriefly described in this section are drawn to scale.

In the drawings, the circle on a symbol representing an electricaldevice (such as a logic gate or an amplifier) is called a bubble, and isused in logic diagrams to indicate a logic negation between the externallogic state and the internal logic state (1 to 0 or vice versa). Thepositive logic convention (i.e., high voltage level=1) is used.

FIG. 1 is a diagram representing (at a high level) one configuration ofa basic electrical controller area network.

FIG. 2 is a diagram representing a typical topology of an electrical CANbus that enables a multiplicity of LRUs to communicate with each other.

FIG. 3A is a diagram showing an electrical CAN bus bit-wise arbitrationbetween two CAN nodes.

FIG. 3B is a physical bit representation of differential voltage levels(CANH and CANL) corresponding to the two inverted logic states (i.e.,dominant and recessive) of a CAN bus.

FIGS. 4A and 4B are hybrid diagrams (combining elements of a blockdiagram and elements of a logical circuit diagram) representing somecomponents of an optical controller area network in a transmit mode(FIG. 4A) and in a receive mode (FIG. 4B) in accordance with oneembodiment.

FIG. 5 is a hybrid diagram representing an optical controller areanetwork comprising a reflective optical star in accordance with oneembodiment.

FIG. 6 is a diagram representing an optical Y-coupler that facilitatesthe propagation of incoming light by internal reflection. The arrowpointing to the right represents optical pulses propagating from atransmit optical subassembly toward a reflective optical coupler; thearrow pointing to the left represents optical pulses propagating fromthe reflective optical coupler toward a receive optical subassemblyassociated with the transmit optical subassembly.

FIG. 7A is a diagram representing an exploded view of some components ofa fiber bundle and reflective optical star assembly in accordance withone proposed example implementation.

FIG. 7B is a diagram representing an isometric view of components of apartly assembled fiber bundle and reflective optical star assembly inaccordance with the proposed example implementation partly depicted inFIG. 7A.

FIG. 7C is a diagram representing an isometric view of components of afully assembled fiber bundle and reflective optical star assembly inaccordance with the proposed example implementation partly depicted inFIGS. 7A and 7B.

FIG. 8 is a diagram representing the geometry of various components ofthe fully assembled fiber bundle and reflective optical star assemblydepicted in FIG. 7C.

Reference will hereinafter be made to the drawings in which similarelements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

Illustrative embodiments of CAN buses comprising a fiber optical networkare described in some detail below. However, not all features of anactual implementation are described in this specification. A personskilled in the art will appreciate that in the development of any suchactual embodiment, numerous implementation-specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem-related and business-related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure.

The technology proposed herein involves the substitution of a fiberoptical network having a reflective optical star in place of anelectrical data bus in a controller area network. Various embodiments ofa fiber optical network for enabling optical communication between LRUs(each LRU incorporating a CAN node) on an aircraft will be described indetail below for the purpose of illustration. However, implementation ofa controller area network comprising a fiber optical network is notlimited solely to the environment of an aircraft, but rather may beutilized in controller area networks onboard other types of vehicles.Also the embodiments of an optical CAN bus disclosed in detail belowhave application in networks of electrical devices other than LRUsprovided that the electrical devices are configured to incorporaterespective CAN nodes.

Technical details concerning the fundamentals and operating principlesof controller area networks have been published. However, for the sakeof adequate disclosure without incorporation by reference, a briefdescription of one implementation of a basic CAN bus will be describedhereinafter with reference to FIG. 1.

FIG. 1 is a diagram representing a controller area network comprising aplurality of N CAN nodes connected to bus lines 2 a and 2 b of a CAN bus2. The CAN bus is terminated at each end by respective resistors 4 a and4 b to prevent signal reflections. Each of the N CAN nodes comprises arespective processor 8, a respective CAN controller 10 and a respectiveCAN transceiver 12. (In alternative implementations, the CAN controllermay be embedded in the processor or embedded in the transceiver. In thelatter instance, the resulting component will be referred to herein as a“CAN controller/transceiver”.) In accordance with the implementationdepicted in FIG. 1, the processor 8 and the CAN transceiver 12communicate with each other (electrically) by way of the CAN controller10. Each CAN transceiver 12 is electrically connected to bus lines 2 aand 2 b of a CAN bus 2 by a pair of stubs 6 a and 6 b. The bus lines 2 aand 2 b and the stubs 6 a and 6 b comprise electrically conductivewires.

In accordance with the CAN communications protocol, each CAN node isable to send and receive messages, but not simultaneously. A message orframe consists primarily of an identifier, which represents the priorityof the message, and a number of data bytes. The message is transmittedserially onto the CAN bus 2 by the CAN transceiver 12 and may bereceived by all CAN nodes. Each CAN node connected to CAN bus 2 waitsfor a prescribed period of inactivity before attempting to send amessage. If there is a collision (i.e., if two nodes try to sendmessages at the same time), the collision is resolved through a bit-wisearbitration, based on a preprogrammed priority of each message in theidentifier field of the message. The message that contains the highestpriority identifier always wins bus access.

Still referring to FIG. 1, CAN communication is bi-directional. Theprocessor 8 decides what the received messages mean and what messages itwants to transmit. Sensors, actuators and control devices (not shown inFIG. 1) can be connected to the processor 8. During transmission, theCAN controller 10 sends the transmit message(s) to the CAN transceiver12, which transmits the bits serially onto the CAN bus 2 when the bus isfree. During reception, the CAN transceiver 12 converts the data streamfrom CAN bus levels to levels that the CAN controller 10 uses. Duringtransmission, the CAN transceiver 12 converts the data stream from theCAN controller 10 to CAN bus levels. More specifically, a driver (notshown in FIG. 1) inside the CAN transceiver 12 converts a digital inputon the TXD terminal to a differential output on the CANH and CANLterminals. A receiver (not shown in FIG. 1) inside the CAN transceiver12 converts the differential signal from the CANH and CANL terminals toa digital output on the RXD terminal. Inside the CAN transceiver 12, theCANH and CANL terminals of the driver are internally tied to thereceiver's input, which enables each transmitting node to constantlymonitor each bit of its own transmission.

The above-described CAN node may be incorporated in various types ofelectrical devices, such a line replaceable unit (LRU). FIG. 2 is adiagram representing a typical topology of an electrical CAN bus thatenables a multiplicity of n LRUs (respectively identified as LRU1, LRU2,. . . , LRUn−1, LRUn, to communicate with each other. Each LRUincorporates a CAN node of the type previously described. In addition,each LRU is connected to a CAN bus 2 by way of a respective LRU-stubconnector 14, a respective stub cable 6, and a respective stub-busconnector 16. The wires of the left portion of the CAN bus 2 may beelectrically connected to wires of the right portion of the CAN bus 2 bya production break connector 18. In accordance with one implementation,each stub-bus connector 16 is a galvanic splice that connects the CANbus 2 to the stub cable 6, one splice for each node on the bus. Thesplicing procedure takes up valuable manufacturing time because theshields have to be separated out, the signal wires are spliced and thenthe shields are connected back in, with heat shrinks and protectivesleeves.

FIGS. 3A and 3B show the CAN bus protocol for an electrical bus where adominant bit overdrives a recessive bit on the bus to achievenondestructive bitwise arbitration. FIG. 3A is a diagram showing anelectrical CAN bus bit-wise arbitration between two CAN nodes. FIG. 3Bis a physical bit representation of differential voltage levels (CANHand CANL) corresponding to the two inverted logic states (i.e., dominantand recessive) of a CAN bus.

As seen in FIG. 3B, the CAN bus 2 has two states during poweredoperation of the device: dominant (a logical 0) and recessive (a logical1). A dominant bus state is when the bus is driven differentially (i.e.,the difference in voltage on the CANH and CANL lines is V_(diff(D))),corresponding to a logic low on the TXD and RXD terminals (shown inFIG. 1) of the CAN controller 10 incorporated in each CAN node. Arecessive bus state is when the CAN bus 2 is biased via high-resistanceinternal input resistors of a receiver (not shown) inside the CANtransceiver 12 (i.e., the difference in voltage on the CANH and CANLlines is V_(diff(R))), corresponding to a logic high on the TXD and RXDterminals.

Only one CAN node can transmit data messages at any given time. If twoCAN nodes try to access the CAN bus 2 simultaneously, the contention isresolved using lossless bit-wise arbitration. Lossless means that theCAN node that wins the arbitration continues to transmit its messagewithout the message being destroyed or corrupted by another CAN node.The CAN arbitration process is handled automatically by the CANcontroller 10. Priority is allocated to a particular CAN node based onan 11-bit identifier which is transmitted by all CAN nodes at the startof each CAN frame. The CAN node with the lowest identifier transmitsmore zeros at the start of the frame, and that node wins the arbitrationor is given the highest priority. A dominant bit always overwrites arecessive bit on a CAN bus.

One example of the CAN bit-wise arbitration is shown in FIG. 3A. Theupper portion represents a sequence of bits being generated by node Aduring a time interval; the middle portion represents a sequence of bitsbeing generated by node B during the same time interval; and the lowerportion represents the sequence of bits transmitted on the CAN bus 2 asa result of the bit-wise arbitration process. Because each nodecontinuously monitors its own transmissions, as node B′s recessive bitis overwritten by node A's higher-priority dominant bit, node B detectsthat the bus state does not match the bit that it transmitted.Therefore, node B halts transmission while node A continues on with itsmessage. Another attempt to transmit the message is made by node B oncethe bus is released by node A. This attempt is successful, as seen inFIG. 3A.

The electrical CAN bus 2 shown in FIG. 2 can be replaced by a passiveoptical CAN bus to reduce weight and labor associated with manufacturingand installation of the bus, and to minimize the many bus configurationsassociated with different data speeds and bus/stub lengths. FIGS. 4A and4B identify some components of an optical controller area network in atransmit mode (FIG. 4A) and in a receive mode (FIG. 4B) in accordancewith one embodiment. The optical controller area network comprises aplurality of electrical CAN nodes operatively coupled to a passivereflective optical coupler (not shown in FIGS. 4A and 4B). Eachelectrical CAN node comprises a respective CAN controller/transceiver 22which is operatively coupled to a respective transmit opticalsubassembly 26 (TOSA) and a respective receive optical subassembly 28(ROSA) by way of a respective signal converter 24. More specifically,the signal converter 24 converts CANH and CANL voltage signals receivedfrom the CAN controller/transceiver 22 to electrical logic bits whichare sent to the transmit optical subassembly 26 and converts electricallogic bits received from the receive optical subassembly 28 to CANH andCANL voltage signals which are sent to the CAN controller/transceiver22.

The signal converter 24 depicted in FIGS. 4A and 4B comprises thefollowing electrical components: (a) a first amplifier 40 havingdifferential input terminals respectively connected to the CANH and CANLterminals of the CAN controller/transceiver 22 and an output terminal;(b) an OR gate 48 having first and second input terminals and an outputterminal; (c) a first AND gate 42 having a first input terminalconnected to the output terminal of the first amplifier 40, a secondinput terminal configured and connected to receive an inverted bit fromthe output terminal of the OR gate 48, and an output terminal connectedto the transmit optical subassembly 26; (d) a second AND gate 44 havinga first input terminal connected to receive an inverted bit from theoutput terminal of the first AND gate 42, a second input terminalconnected to the receive optical subassembly 28, and an output terminalconnected to the first input terminal of the OR gate 48; (e) a secondamplifier 46 having an input terminal connected to the output terminalof the second AND gate 44, a first output terminal connected to the CANHterminal of the CAN controller/transceiver 22, and a second outputterminal configured and connected output an inverted voltage signal tothe CANL terminal of the CAN controller/transceiver 22; and (f) a shiftregister 50 having an input terminal connected to the output terminal ofthe second AND gate 44 and an output terminal connected to the secondinput terminal of the OR gate 48. The shift register 50 provides apropagation time delay for the signal in the internal loop from theoutput terminal of the second AND gate 44 to the second input terminalof the first AND gate 42.

FIGS. 4A and 4B show how the dominant/recessive bit behavior in anelectrical CAN bus is achieved in the optical domain in accordance withone embodiment. A recessive state output from a node means no opticalpulse is transmitted. A dominant state output from a node means anoptical pulse is transmitted. The bit output by the AND gate 42 isinverted and fed back to the AND gate 44. The bit output by the AND gate44 is then looped back to the AND gate 42 via the OR gate 48 to lock outother CAN controller/transceivers (see FIG. 5) that are connected andconfigured to communicate with CAN controller/transceiver 22 via thetransmit and receive optical subassemblies 26, 28. Collision fromoptical transmitters with different numbers of dominant state bits(i.e., optical pulses) is similar to how a dominant CAN signal issupposed to behave, i.e., the transmitter that transmits more successiveoptical pulses at the start of a CAN frame wins the arbitration andcontinues to transmit its message.

The configuration of components depicted in FIG. 4A can be used toconnect multiple CAN controller/transceivers to a single reflectiveoptical star. FIG. 5 depicts a multiplicity of CANcontroller/transceivers 22 a through 22 n (where n is an integer equalto three or more) operatively coupled to a reflective optical star 32.CAN controller/transceiver 22 a is operatively coupled to reflectiveoptical star 32 by way of a signal converter 24 a, a transmit opticalsubassembly 26 a, a receive optical subassembly 28 a, and an opticalY-coupler 30 a having two branches which are respectively opticallycoupled to the transmit and receive optical subassemblies 26 a, 28 a.CAN controller/transceiver 22 b is operatively coupled to reflectiveoptical star 32 by way of a signal converter 24 b, a transmit opticalsubassembly 26 b, a receive optical subassembly 28 b, and an opticalY-coupler 30 b having two branches which are respectively opticallycoupled to the transmit and receive optical subassemblies 26 b, 28 b.CAN controller/transceiver 22 n is operatively coupled to reflectiveoptical star 32 by way of a signal converter 24 n, a transmit opticalsubassembly 26 n, a receive optical subassembly 28 n, and an opticalY-coupler 30 n having two branches which are respectively opticallycoupled to the transmit and receive optical subassemblies 26 n, 28 n. Inaccordance with the embodiment depicted in FIG. 5, each of the signalconverters 24 a through 24 n comprises the components depicted insidethe dashed rectangle in FIG. 4. In addition, the optical Y-couplers areoptically coupled to the transmit and receive optical subassemblies andthe reflective optical star by way of plastic optical fibers (POF inFIG. 5).

An optical fiber is a cylindrical dielectric waveguide that transmitslight along its axis. The fiber consists of a transparent coresurrounded by a transparent cladding layer (hereinafter “cladding”),both of which are made of dielectric materials. Light is kept in thecore by the phenomenon of total internal reflection. To confine theoptical signal in the core, the refractive index of the core is greaterthan that of the cladding. The boundary between the core and claddingmay either be abrupt, as in step-index fiber, or gradual, as ingraded-index fiber. The embodiments disclosed herein employ plasticoptical fibers. Plastic optical fibers have high transmission capacity,excellent immunity to electromagnetic interference-induced noise, lightweight, high mechanical strength, and outstanding flexibility. Plasticoptical fibers are also larger in diameter as compared to glass opticalfibers. Due to their larger diameters, plastic optical fibers havegreater tolerance for fiber misalignment than glass optical fibers have.Because of this large misalignment tolerance, plastic opticalfiber-based networks have lower maintenance and installation costs. Inaerospace platforms, plastic optical fibers also greatly reduce the costof connectors and transceiver components used in an avionics network. Inalternative embodiments, glass optical fibers can be used in place ofplastic optical fibers.

In accordance with the embodiments disclosed herein, the reflectiveoptical star 32 is operatively coupled to the CANcontroller/transceivers 22 a through 22 n with only one plastic opticalfiber (POF) per CAN controller/transceiver using optical Y-couplers 30 athrough 30 n. Thus, by using reflective optical star 32, any dominantsignal (an optical pulse) on the CAN bus can be seen by the sender's ownreceiver and also by all other receivers on the CAN bus, which overridesall recessive signal senders.

FIG. 6 is a diagram representing an optical Y-coupler 30 in accordanceone embodiment. The optical Y-coupler 30 comprises three plastic opticalfibers 52, 54 and 56. Although not depicted in FIG. 6, the end faces 52a and 54 a of plastic optical fibers 52 and 54 will be bonded andoptically coupled to the end face 56 a of plastic optical fiber 56. Theoptical Y-coupler 30 is designed to facilitate the propagation ofincoming light by internal reflection. The arrow T pointing to the rightin FIG. 6 represents light propagating from left to right throughoptical Y-coupler 30 (e.g., from a transmit optical subassembly 26 to areflective optical star 32), whereas the arrow R pointing to the leftrepresents light propagating from right to left (e.g., from thereflective optical star 32 to a receive optical subassembly 28associated with the transmit optical subassembly 26). The opticalY-coupler 30 enables single-fiber connection of a CAN node to thereflective optical star 32. In accordance with one example of a proposedimplementation, a single 1-mm-diameter plastic optical fiber (not shownin FIG. 6) is used for bidirectional data transmission from plasticoptical fiber 56 of each optical Y-coupler 30 to the reflective opticalstar 32, as seen in FIG. 5. Similarly the transmit and receive branchesformed by plastic optical fibers 52 and 54 are optically coupled torespective 1-mm-diameter plastic optical fiber stubs which connect tothe transmit and receive optical subassemblies 26 and 28 respectively,as seen in FIG. 5.

In accordance with the above-described proposed implementation, theplastic optical fibers 52, 54 and 56 each have a diameter of 1 mm exceptalong respective end sections of plastic optical fibers 52, 54. Each ofthe plastic optical fibers 52 and 54 comprise respective end sectionswhere fiber material has been removed to form respective planar facesand respective semicircular end faces 52 a and 54 a. The end sectionsbegin where the circular cross sections of plastic optical fibers 52 and54 transition to non-circular and terminate at the semicircular endfaces 52 a and 54 a respectively. More specifically, the end section ofplastic optical fiber 52 is shaped to form a first side face thatintersects and is perpendicular to end face 52 a, while the end sectionof plastic optical fiber 54 is shaped to form a second side face thatintersects and is perpendicular to end face 54 a. These side faces arebonded to opposite surfaces of a thin layer of reflective material 58,such as silver. The thin layer of reflective material 58 preventscross-talk between the respective end sections of the plastic opticalfibers 52 and 54. The semicircular end faces 52 a and 54 a of theplastic optical fibers 52 and 54 combine to form a circular end facethat is bonded and optically coupled to a circular end face 56 a of theplastic optical fiber 56 by a layer of index matching epoxy (not shownin FIG. 6). This index matching epoxy eliminates the back reflection atthe semicircular end face 54 a that may cause cross-talk from thetransmit optical subassembly 26 to the associated receive opticalsubassembly 28 of the same CAN node.

The construction of a POF bundle and reflective optical star assembly inaccordance with one proposed example implementation will now bedescribed with reference to FIGS. 7A through 7C.

FIG. 7A is an exploded view showing the following components: a fiberbundle 60 comprising a multiplicity of (in this example, seven) plasticoptical fibers 62 surrounded by protective jackets 64 (except for theraw ends visible in FIG. 7A); a fiber bundle sleeve 66 in which an endportion of the fiber bundle 60 will be inserted during assembly; anoptical mixing rod 68 having a hexagonal cross-sectional profile; and amixing rod sleeve 70 in which the optical mixing rod 68 will be insertedduring assembly. A first end face 65 of the optical mixing rod 66 ispolished and will be optically coupled to the end faces of the sevenplastic optical fibers 64; a second end face of the optical mixing rod66 is coated with a thin film of reflective material that forms a mirror67.

FIG. 7B is an isometric view of a partly assembled fiber bundle andreflective optical star assembly having the components depicted in FIG.7A. The end portion of the fiber bundle 60 (which end portion includesthe raw ends of the plastic optical fibers 64) is surrounded by thefiber bundle sleeve 66. The axial portion of the fiber bundle sleeve 66that receives the raw ends of the plastic optical fibers 64 has atapered internal surface in the axial portion of the fiber bundle sleeve66 which constrains the raw ends of the plastic optical fibers 64. Theoptical mixing rod 68 is surrounded by the mixing rod sleeve 70. FIG. 7Bshows that the fiber bundle sleeve 66 and the mixing rod sleeve 70separated by a gap. In the final assembly, the end faces of the fiberbundle sleeve 66 and mixing rod sleeve 70 will abut each other and theend faces of the plastic optical fibers 64 will be bonded and opticallycoupled to end face 65 of the optical mixing rod 66.

FIG. 7C shows an isometric view of components of a fully assembled fiberbundle and reflective optical star assembly in accordance with theproposed example implementation partly depicted in FIGS. 7A and 7B. Inthe final assembly, the end faces of fiber bundle sleeve 66 and mixingrod sleeve 70 abut each other. In addition, the end faces of plasticoptical fibers 64 are bonded and optically coupled to end face 65 of theoptical mixing rod 66 by a layer of index matching epoxy (not visible inFIG. 7C). The function of the optical mixing rod 66 is to mix all theelectro-magnetic modes that propagate from any one of the seven plasticoptical fibers 64 so that, following reflection by the mirror 67, thereflected electromagnetic radiation will be uniformly distributed to allseven plastic optical fibers 64. The fiber bundle sleeve 66 and mixingrod sleeve 70 are in turn surrounded by a star outer housing 72.

FIG. 8 is a diagram representing the geometry of various components ofthe fully assembled fiber bundle and reflective optical star assemblydepicted in FIG. 7C. The outer diameters of the fiber bundle sleeve 66and mixing rod sleeve 70 are equal. The star outer housing 72 is acircular cylinder having an inner diameter greater than the outerdiameter of the sleeves. The star outer housing 72 is provided with anaccess hole 74 for the injection of adhesive between the star outerhousing 72 and the sleeves. The fiber bundle sleeve 66 comprises twocircular cylindrical sections having relatively smaller and relativelylarger inner diameters respectively. The circular cylindrical sectionhaving the relatively larger inner diameter surrounds a jacketed portionof the fiber bundle 60, whereas the circular cylindrical section havingthe relatively smaller inner diameter surrounds the raw ends of theplastic optical fibers 64. In one proposed implementation, the plasticoptical fibers 64 have an outer diameter of 1 mm. The inner diameter ofthe mixing rod sleeve 70 is indicated by the dashed circle in FIG. 8.The cross-sectional profile of the optical mixing rod 66 is indicated bythe dashed hexagon inside the dashed circle in FIG. 8. The location ofthe end faces of the seven plastic optical fibers 64 inside the dashedhexagon represents the fact that all of the fibers are optically coupledto the first end face 65 of the optical mixing rod 66.

In accordance with one example proposed implementation, each signalconverter 24 (see FIG. 4A) is electrically coupled to a respectiveoptical-electrical media converter. Each optical-electrical mediaconverter comprises: a respective transmitter that has a laser forconverting electrical signals received from a respective signalconverter 24 into optical signals to be sent to the optical mixing rod66 of the reflective optical star 32; and a respective receiver that hasa photodetector that converts optical signals received from the opticalmixing rod 66 into electrical signals to be sent to the respectivesignal converter 24.

CAN node components that are suitable for use in the applicationsdisclosed above are commercially available from Texas Instruments Inc.,(such as the SN65HVD26x CAN transceiver).

The optical CAN bus disclosed herein uses a single fiber for transmitand receive, a single wavelength for transmit and receive, and a passivereflective optical star to avoid single-point electronic failure as inan active star. In accordance with one proposed implementation, a single1-mm-diameter POF is used for bidirectional data transmission from oneLRU to the reflective optical star by way of an optical Y-coupler withhigh isolation between transmit and receive branches which are alsooptically coupled to a pair of 1-mm-diameter POF stubs. The assemblyprovides a simple optical fiber bus with no connectors between thepassive reflective optical star and POF stubs, no splices and noterminators. The resulting optical CAN bus functions independent ofselected bus speed and independent of the distance from the reflectiveoptical star to the CAN nodes inside the LRUs.

The optical CAN bus disclosed herein can be employed on any mobileplatform (car, tank, airplane, helicopter, spaceship, etc.) or fixedplatform (industrial machinery, etc.) to eliminate EME and reduceweight, size and manufacturing time associated with the limitations ofelectrical CAN buses.

While optical networking systems have been described with reference tovarious embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the teachings herein. Inaddition, many modifications may be made to adapt the concepts andreductions to practice disclosed herein to a particular situation.Accordingly, it is intended that the subject matter covered by theclaims not be limited to the disclosed embodiments.

As used in the claims, the term “optical waveguide” includes at leastone of the following types of elements configured to guideelectromagnetic radiation propagating through the waveguide: opticalfibers, optical connectors, optical Y-couplers and optical mixing rods.

In addition, the corresponding structure (disclosed hereinabove) thatperforms the function of “converting differential electrical signals tooptical pulses” (as recited in the claims) comprises the signalconverter 24 and the transmit optical subassembly 26 and equivalentsthereof; and the corresponding structure (disclosed hereinabove) thatperforms the function of “converting optical pulses to differentialelectrical signals” (as recited in the claims) comprises the signalconverter 24 and the receive optical subassembly 28 and equivalentsthereof.

1. A data communications system comprising: a plurality of controllerarea network nodes which operate electrically; a plurality of signalconverters electrically coupled to respective controller area networknodes of the plurality of controller area network nodes, each signalconverter comprising electrical circuitry that converts differentialsignals to digital signals and vice versa; a plurality of transmitoptical subassemblies electrically coupled to respective signalconverters of the plurality of signal converters, each transmit opticalsubassembly comprising a respective transmitter that converts digitalsignals from a respective signal converter to optical pulses; aplurality of receive optical subassemblies electrically coupled torespective signal converters of the plurality of signal converters, eachreceive optical subassembly comprising a respective receiver thatconverts optical pulses to digital signals which are sent to arespective signal converter; and a fiber optical network opticallycoupled to the transmitters and receivers for enabling the plurality ofcontroller area network nodes to communicate with each other, whereinthe fiber optical network comprises a reflective optical star.
 2. Thesystem as recited in claim 1, wherein the fiber optical network furthercomprises a plurality of optical Y-couplers optically coupled to thereflective optical star, each optical Y-coupler comprising transmit andreceive branches which are respectively optically coupled to thetransmitter and receiver associated with a respective signal converter.3. The system as recited in claim 2, wherein the transmit branch of eachoptical Y-coupler comprises a first optical fiber having a first sideface, the receive branch of each optical Y-coupler comprises a secondoptical fiber having a second side face that confronts the first sideface, and each optical Y-coupler further comprises a layer of reflectivematerial disposed between the first and second side faces of thetransmit and receive branches.
 4. The system as recited in claim 3,wherein the first optical fiber has a first end face, the second opticalfiber has a second end face, and each optical Y-coupler furthercomprises a third optical fiber having an end face which is opticallycoupled to the first and second end faces.
 5. The system as recited inclaim 3, wherein each optical Y-coupler further comprises a layer ofindex matching epoxy disposed between the first and third end faces andbetween the second and third end faces.
 6. The system as recited inclaim 1, wherein the reflective star coupler comprises an optical mixingrod and a mirror disposed at one end of the optical mixing rod.
 7. Thesystem as recited in claim 1, wherein each of the plurality ofcontroller area network nodes comprises a respective controller areanetwork controller and a respective controller area network transceiverelectrically coupled to the respective controller area networkcontroller, the controller area network controllers being configured tocommunicate using bitwise arbitration.
 8. The system as recited in claim7, wherein each signal converter comprises: a first amplifier havingdifferential input terminals respectively connected to CANH and CANLterminals of a respective controller area network transceiver and anoutput terminal; an OR gate having first and second input terminals andan output terminal; a first AND gate having a first input terminalconnected to the output terminal of the first amplifier, a second inputterminal configured and connected to receive an inverted bit from theoutput terminal of the OR gate, and an output terminal connected to arespective transmit optical subassembly; a second AND gate having afirst input terminal connected to receive an inverted bit from theoutput terminal of the first AND gate, a second input terminal connectedto a respective receive optical subassembly, and an output terminalconnected to the first input terminal of the OR gate; and a secondamplifier having an input terminal connected to the output terminal ofthe second AND gate, a first output terminal connected to the CANHterminal of the respective controller area network transceiver, and asecond output terminal configured and connected output an invertedvoltage signal to the CANL terminal of the respective controller areanetwork transceiver.
 9. The system as recited in claim 8, furthercomprising a shift register having an input terminal connected to theoutput terminal of the second AND gate and an output terminal connectedto the second input terminal of the OR gate.
 10. A data communicationssystem comprising: a plurality of electrical devices configured forsending and receiving electrical signals representing data, wherein eachof the electrical devices comprises a respective controller area networkcontroller configured to broadcast messages using bitwise arbitration todetermine message priority, and a respective controller area networktransceiver electrically coupled to the respective controller areanetwork controller; a plurality of signal converters electricallycoupled to respective controller area network transceivers of theplurality of controller area network transceivers, each signal convertercomprising electrical circuitry that converts differential signals todigital signals and vice versa; a plurality of transmit opticalsubassemblies electrically coupled to respective signal converters ofthe plurality of signal converters, each transmit optical subassemblycomprising a respective transmitter that converts digital signals from arespective signal converter to optical pulses; a plurality of receiveoptical subassemblies electrically coupled to respective signalconverters of the plurality of signal converters, each receive opticalsubassembly comprising a respective receiver that converts opticalpulses to digital signals which are sent to a respective signalconverter; and a fiber optical network optically coupled to thetransmitters and receivers for enabling the plurality of controller areanetwork nodes to communicate with each other, wherein the fiber opticalnetwork comprises a reflective optical star.
 11. The system as recitedin claim 10, wherein the fiber optical network further comprises aplurality of optical Y-couplers optically coupled to the reflectiveoptical star, each optical Y-coupler comprising transmit and receivebranches which are respectively optically coupled to the transmitter andreceiver associated with a respective signal converter.
 12. The systemas recited in claim 10, wherein the transmit branch of each opticalY-coupler comprises a first optical fiber having a first side face, thereceive branch of each optical Y-coupler comprises a second opticalfiber having a second side face that confronts the first side face, andeach optical Y-coupler further comprises a layer of reflective materialdisposed between the first and second side faces of the transmit andreceive branches.
 13. The system as recited in claim 10, wherein thereflective star coupler comprises an optical mixing rod and a mirrordisposed at one end of the optical mixing rod.
 14. The system as recitedin claim 10, wherein each of the plurality of electrical devices is arespective line replaceable unit.
 15. A data communications systemcomprising: a plurality of electrical devices configured for sending andreceiving electrical signals representing data, wherein each of theelectrical devices comprises a respective controller area networkcontroller configured to broadcast messages using bitwise arbitration todetermine message priority, and a respective controller area networktransceiver electrically coupled to the respective controller areanetwork controller; means for converting differential electrical signalsto optical pulses; means for converting optical pulses to differentialelectrical signals; and a fiber optical network comprising a reflectiveoptical star and a multiplicity of optical wave guides that opticallycouple the reflective optical star to the means for convertingdifferential electrical signals to optical pulses and to the means forconverting optical pulses to differential electrical signals.
 16. Thesystem as recited in claim 15, wherein the multiplicity of optical waveguides further comprise a plurality of optical Y-couplers opticallycoupled to the reflective optical star, each optical Y-couplercomprising transmit and receive branches which are respectivelyoptically coupled to a transmitter associated with a respective meansfor converting differential electrical signals to optical pulses and areceiver associated with a respective means for converting opticalpulses to differential electrical signals.
 17. The system as recited inclaim 16, wherein the transmit branch of each optical Y-couplercomprises a first optical fiber having a first side face, the receivebranch of each optical Y-coupler comprises a second optical fiber havinga second side face that confronts the first side face, and each opticalY-coupler further comprises a layer of reflective material disposedbetween the first and second side faces of the transmit and receivebranches.
 18. The system as recited in claim 15, wherein the reflectivestar coupler comprises an optical mixing rod and a mirror disposed atone end of the optical mixing rod.
 19. The system as recited in claim15, wherein each of the plurality of electrical devices is a respectiveline replaceable unit.
 20. A method for controller area networkcommunication between a plurality of nodes, comprising: broadcasting amessage from one of the plurality of nodes, the message comprisingtransmit differential electrical signals representing a sequence of bitsin accordance with a communication protocol that employs bitwisearbitration in response to collision detection; converting the transmitdifferential electrical signals to optical pulses; guiding the opticalpulses toward and into a reflective optical star; reflecting the opticalpulses inside the reflective optical star; guiding reflected opticalpulses toward the plurality of nodes; converting reflected opticalpulses to receive differential electrical signals; and receiving thereceive differential electrical signals at each node.